1. Field of the Invention
The present invention relates to explosion-proof porcelain housings for gas-filled insulating apparatuses, and a process for producing such porcelain housings. More particularly, the invention relates to explosion-proof porcelain housings for gas-filled insulating apparatuses, which is adapted to prevent broken pieces thereof from being scattered if the porcelain housing is broken due to the pressure of a gas inside the gas-filled insulating apparatus. The invention also relates to a process for producing such explosion-proof porcelain housings.
2. Related Art Statement
For attaining the above purpose, explosion-proof porcelain housings in which a film made of an insulating material formed on an inner surface of a porcelain housing body are formerly known. A typical porcelain housing includes a single layer of a synthetic resin or an elastomer bonded to an inner surface of a porcelain housing body.
However, as to this explosion-proof porcelain housing having a single film layer bonded thereto, as shown in FIG. 3, when the porcelain housing body 11 is cracked for some reason, an internal pressure is abruptly applied to circumferentially expand the film 12 at a cracked portion. That is, since the film 12 is bonded to the porcelain housing body 11, circumferential stresses are concentrated on the outer side surface of the film 12 at the cracked portion 13 of the porcelain housing body 11. The distribution of circumferential stresses is shown in FIG. 3. Since the film 12 is readily torn by this concentration of the stresses, a sufficient explosion-proof effect cannot be obtained.
In order to solve the defects of such a conventional explosion-proof porcelain housing having a single film integrated with the porcelain housing body, NGK Insulators, Ltd. formerly developed an explosion-proof porcelain housing in which films made of two kinds of materials, respectively, are formed on an inner surface of a porcelain housing body in a non-bonded state as shown in Japanese patent application Laid-open No. 61-264,612. However, if such an explosion-proof porcelain housing is cracked due to some cause, since neither film is bound to the porcelain housing body, the internal pressure acts upon the entirety of the films. As a result, the film expands in the form of a balloon, such that the films are stretched and become thinner. Since intensity of stresses occurring in the film due to the internal pressure are proportional to the diameter, and are inversely proportional to the thickness, the films are further expanded with the stresses and are finally broken. In addition, since neither film is bonded to the porcelain housing body, broken pieces of the porcelain housing body are scattered in all directions. Therefore, sufficient explosion-proof effect cannot be expected.